If you are looking at buying a tri specific bike then I believe that there is only one maybe two companies (trek for sure) that actually sell a women specific design. There is such a wide variety in geometry of tri frames out there that I don't see the benefit in a company marketing a women specific design. They key component is the fit. If you are 5'11" a 55/56 seems a little big unless your torso is long. Perhaps on a road bike you would be a 55/56 but I would think that on a tri bike you would want a slightly smaller frame ie. 54. I wouldn't call it a man's specific bike as most companies don't call it that. Have a look at the Pro women most ride a normal bike some ride custom bikes.

Bottom line it is the size and geometry of the frame that matters. There is no set standard for bike design and you will see a vast array of geometries. Others on here know significantly more than I.